The overall goal of this project is to gain a better understanding of the determinants of pulmonary autoregulation of ventilation-perfusion relationships. In the first year of the project we have demonstrated that interlobar collateral ventilation can prevent lobar absorption atelectasis and lobar hypoxic pulmonary vasoconstriction ( J.Appl.Physiol.) we have also found that increasing the amount of lung that is hypoxic does not cause an increased amount of hypoxic pulmonary vasoconstriction (Anesthesiology). In the second and third years of this project we plan to (1) determine the mechanism of blood flow decrease through atelectatic lung; (2) vary the oxygen tension in the phase in the normal fraction of lung from 100 percent 02 to room air and observe the effect on the blood flow to the nitrogen ventilated left lower lobe; (3) infuse systemically vasodilator drugs (sodium nitroprusside, nitroglycerin, pentolinium and trimethaphon) and examine effects on blood flow to hypoxic compartment in a lung that has high pulmonary vascular pressures; and (4) raise alveolar and end-expiratory pressure and lung volume in normal lung with and without similar manipulations in a test hypoxic lobe and examine test lobe hypoxic pulmonary vasoconstriction.